1. Field of the Invention
The present invention relates to a direct current to direct current (hereinafter, “DC-to-DC”) voltage converter, and more particularly, to a DC-to-DC voltage converter capable of achieving duty control and switching frequency stabilization using switching frequency detection.
2. Discussion of Related Art
A conventional DC-to-DC voltage converter provides an input supply voltage as a converted output voltage having an appropriate level and such DC-to-DC voltage converter has been used in various electronic devices such as a cellular phone, a radio frequency (RF) communication device, etc. In general, a DC-to-DC voltage converter requires a method of regulating its output voltage to have a target level in consideration of changes of output loading condition and a ratio between input and output voltages.
Such output voltage regulation method can be performed by a voltage mode control (VMC) method and/or a current mode control (CMC) method in a linear control type.
The voltage mode control method is for controlling an output voltage by directly feeding a level of a converted output voltage back to driving circuits. Since the voltage mode control method performs in a low response speed with respect to change of the output voltage, it has a disadvantage of a great voltage drop while the load is changed significantly.
And, the current mode control method is for regulating an output voltage by detecting current change of an upper or lower switch. The current mode control method has advantages of having a high response speed with respect to the change of the converted output voltage, but disadvantages such that it requires complicated current sensing circuit to ensure stability.
Besides the linear control type, time or frequency based nonlinear control methods also have been developed. The nonlinear control methods would be advantageous since they have a high response speed with respect to the change of the converted output voltage and they are easy to be implemented in a circuit. However, the conventional nonlinear control methods are disadvantageous such that a switching frequency varies along the input and output conditions. This switching frequency variation causes difficulty in handling an electromagnetic interference (EMI) phenomenon and degradation of system performance when the switching frequency moves into a low frequency band. Recently some other technics are introduced in the nonlinear control methods, but the control is still very sensitive to external conditions.